Set Channel’s alpha with masks does not work if set to same layer
The Set Channel effect does not seem to respect masks for the alpha source anymore, if the layer refers to itself.
When „Source Layer 4“ points to the same layer it is applied to, with the dropdown set to „Masks“, the effect would apply the layer’s alpha, but only within the layer’s masks. Now, it does not respect the masks at all and won’t apply the alpha.
Simple Setup: red shape layer with a mask, „Solid Composite“ with green color applied, then „Set Channels“ with „Source Layer 4“ set to the same shape layer, using „Masks“. See attached images for what the setup did before and what it is doing now.
The effect still seems to be working fine, as long as the alpha refers to another layer.
The issue might be related to the recent revision of the Set Channel effect with the may 2020 update: https://helpx.adobe.com/after-effects/using/whats-new/2020-1.html#Otherenhancements

The fix for this issue was included with After Effects 2021 ( version 18.4 ).
Thanks again for helping make us aware of this issue and please let us know if it is not working correctly in versions later than After Effects 18.4.
- John, After Effects Engineering Team
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Max commented
Can’t say when it happened between the numerous releases since the issue first occurred, but it seems to be fixed by now. Thanks to whoever was in charge of that :)
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Chelsie Smith commented
Any update on this? I'm also having the same issue.
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Hi Max,
Thank you for reporting this issue, we are able to reproduce it on our end as well. Thank you for the reference to the update to Set Channels as well, that is very helpful. We are now tracking this issue internally as DVAAE-4215869 and this thread will be kept updated.
Thanks again and apologies for any inconvenience this change has caused,
- John, After Effects Engineering Team